I currently have an iSonic 133 (2007) for light wind days. I am using it with sails:
- Code Red 8.3 (2008) with 48 cm fin
- Code Red 10.0 (2007) with 52 and 55 cm fins

In my experience there is almost no difference in early planing between the 8.3 and 10.0 for me. But the 8.3 has a far better high end, so I end up mostly using the 8.3 even on the light days.

What is your opinion, is this because I am relatively ligh weight at 70kg / 180cm? Is using such a big sail useless for me because of my weight? Or do I have some problems with my technique or setup with the 10.0?

I experienced much the same thing, but on less racy equipment. For years I had a 145 liter free ride board and a 9.5 cambered sail. There were about five or six days each year when I would go out on a nice low wind day but got completely overpowered when the wind came up suddenly--down in the water, hoping the wind would die overpowered. I also had a 120 liter freeride board that would carry an 8.5. I started noticing the 120 and 8.5 planed just as early as the 145 and 9.5 combination, maybe even earlier (because I could pump it on a plane earlier)--and I could handle the smaller combination better when the wind came up suddenly. The sails were the same make and same brand, so it was not due to differences in sail quality. The thing I concluded is that, for my 155 pounds (70 kg), putting up a bigger sail does not help. I think there is a point of diminishing returns and I had gone past it with the 9.5. And since I didn't need a board that would carry a 9.5, why keep it? It made my windsurfing life a lot easier.

I believe in the theory about diminishing returns, and a 10 m2 sail on a slalom board with a 70 kg rider starts to get there.
But it may also be that the 8.3 is a slalom sail, while the 10 m2 is a highwind formula sail that was never designed for any early planing!

I believe in the theory about diminishing returns, and a 10 m2 sail on a slalom board with a 70 kg rider starts to get there.
But it may also be that the 8.3 is a slalom sail, while the 10 m2 is a highwind formula sail that was never designed for any early planing!

I agree, a big slalom sail will have more power than a small formula sail.

At 70 kgs a 8.5 will get you going in very light winds with a 133 Isonic. The wider the board, generally the bigger the sail you need get it unstuck from the water and planing (diminishin returns...). Bigger sails also will push the nose down making it harder to get going. Hence formula gear goes with 1m wide boards. The entry to a formula sail is narrower, the sail flatter, all designed to upwind, and offcourse dead downwind. So at 70kg, its not your technique, its the sail.

Being 70kgs you may also find a 7.8 plus I107 will get you going pretty damm early also, but with a much better top end an ride. I'm 95kg and I107 is my biggest board.

the 10-oh is manageable to about 18 knots
after that it becomes work and flys - it can do more - it's me

i am purchasing a used SB Carve 121 to experience this lighter feeling
my understanding is the sweet spot for this board is 5.5 to about 7.5
not sure this would plane in the same wind ie 12 knots

in summary : diminishing returns depends on where you live. when i go out with my 10-oh, there are usually few or no windsurfers. kiters maybe. and the sail is not so large as to require a massive fin - works with a 53 cm race or 47cm weed fin

so far i have NOT regretted the purchase of my 10-oh, 520 mast and carbon boom. it gets used about 45 % of my outings (12 to 18 knots) and my 8.5 another 45 % (15 to 20 knots range)

no idea how a 107 liter board could ever be my largest board
light wind is the majority of my sailing
with 95 kilos 6.0 and 7.0 are used only 10 % of the time !!!!

my w/s buddy who is 105 kilos and 6'4" stops at 8.5
says 10.0 is too heavy and not worth the investment

Sam, you mention a 48 cm fin with your 8.3 which is a good combination for lightwind. Normally you should be able to plane a couple of knots earlier with 10 sqm but the difference may be hardly noticeable unless you have much more volume under your feet. Have you tried the 52 cm fin with your 8.3? This is a bit oversized but may work even better.

I havent't tried the 52 cm fin with the iSonic 133, I have thought that I might be a little on the large side for me, because the 48 cm fin begins to feel also a little big when properly powered.

I was wondering, is there a practical minimum limit in windspeed (around 8-9 knots?) that modern slalom equipment can be used? I guess for the slalom boards there is a minimum boardspeed that is required to keep the board planing (about 15 knots?). So could it be the case that when the wind is too light, the angle of the actual wind in the sail will turn too forward (like trying to point too high) making it impossible to keep the required 15 knots of boardspeed. And in that case switching to a bigger sail does not help much, becuause the liming factor is the angle of the wind, not the power in the sail? So maybe I have already reached that minimum windspeed with 8.3, and swithing to 10.0 does therefore not help?

Then on which board are you using Code Red 10.0 with 52 and 55 cm fins? There is certainly a threshold where a slalom board will stop planning for the reasons you mention. However I don't think you have reached it. You may plane in ~11 knots with 8.3 and ~9 knots with 10 sqm (on a bigger board?) and a lighter sailor may plane even earlier.

Also, 10 knot cold winds have a lot more grunt than 10 knot warm winds, IE In the UK, 15k to 20 k winter/Autum winds for me means a 7m, but 15k to 20k summer winds would be an 8m for me. so location is an important consideration.

I use a 9m and 10m on my iS137. I dont normally nee to get the 10m out as the 9m is usually plenty if the wind does not drop below 10k. The 10m is more for 8k to 12k. I prefer the 9m as this does 10k to 20k easily.

Main difference between a 10m and 9m is that you will feel and have more power to pump, sail deeper and point higher. Speed will not always be higher with the bigger sail because of the extra drag/weight